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Steam Pressure Reducing Stations

December 14, 2010 - 1:06pm  Anonymous (not verified)

Many industrial plants produce high-pressure steam for process work.In many of these plants, there is excess steam capacity available that can be utilized for other purposes, such as space heating, water heating, etc.Process steam is often generated at a higher pressure than can be used for the other purposes.Whatever the needs for lower pressure steam, a pressure reducing station will be required.

A pressure reducing station is more than just a reducing valve fed off a steam main.A properly designed and installed pressure reducing station takes into consideration velocity, good piping practices, and safety.

After selecting the correct pressure-reducing valve (see Info-Tec 28), the next step in designing the complete reducing station is to pipe size for velocity.The pressure difference created by the reducing valve creates higher steam velocity across the seat of the reducing valve.Sonic velocity will occur.The pipes going into or out of the reducing valve cannot tolerate this.Erosion of materials and excessive noise will be generated.It is good practice to limit steam velocity to between 4000 and 6000 feet per minute to prevent erosion and noise.Properly sized pipes also allow the downstream pressure sensing line to function.Excessive steam velocity passing the sensing line connection will cause inaccurate variable pressures to be sensed as the load varies.Downstream pressure cannot be maintained.

There is a formula to find steam velocity in pipes.Velocity = 2.4 x flow in lbs/hr x sp. vol. in cu. ft./lb at the flowing pressure over the internal area of the pipe in sq. inches.

This formula has been incorporated into an easy-to-use chart, so there is no need to make laborious calculations.See Figure1.

Figure 1.

Example 1, using the chart:

A pressure-reducing valve has been sized for reducing 100-psig-inlet pressure to 25-psig.Capacity is 1000 lbs/hr. Find the upstream and downstream pipe sizes for reasonable quiet steam velocity.

For upstream pipe size, enter the chart at “A,” 1000 lbs/hr.Go horizontally to where the 1000 lbs/hr line intersects 100 psig, point “B.”From point “B” go vertically to intersect with the first pipe size line inside the shaded area, the 4000 to 6000 FPM area.This is point “C,” 1-1/2” schedule 40-iron pipe.If point “C” is extended horizontally to the right side velocity scale, you see that the actual velocity is about 4800 ft./min., point “D.”

For downstream piping, enter the chart once again at “A.”Go horizontally to the downstream pressure of 25 psig, point “E.”Go vertically from “C” to intersect with the first pipe size line in the shaded area, point “F,” 2-1/2” schedule 40-iron pipe.Actual velocity shown at “G” is about 5500 FPM.

The upstream piping to the pressure regulator should be 1-1/2”, the downstream piping out of the regulator should be 2-1/2.”(If schedule 80 iron pipe is going to be used, note the multiplying factor in the chart.)

One more example before expanding on proper piping practice.

Example 2:

Inlet pressure:100 psig

Reduced pressure:10 psig

Flow rate:6000 lbs/hr

Pressure regulator selected:2-1/2” (Info-Tec 28)

Upstream piping size for schedule 40 pipe.Enter chart at 6000 lbs/hr and intersect at 100-lb line.Vertically to first pipe size line in shaded area, 4” pipe.

Downstream Pipe:Enter chart at 6000 lbs/hr.Go to the 10-psig line and vertically intersect it at the 8” pipe.The lengths of these up and downstream pipes are also important.Upstream pipe should be a minimum of six pipe diameters long.Downstream pipe a minimum of 10 pipe diameters long.Using example 2, these pipes should be at least two feet long for upstream, and 80 inches long for downstream.These are minimum lengths and sizes.You can always go longer and larger (cost could become a factor).

Good steam piping practice involves the use of eccentric couplings, mounting accessories so no traps are formed, and proper pitching of lines.

Figure 2 is a schematic diagram of a typical pressure reducing station.Study Figure 2.Note the use of eccentric couplings.These prevent formation of water pockets where condensate could collect.A Y pipe strainer is installed laying on its side for the same reason, or it would have to be drained with a trap.Manual ball valves are used as shut-offs to facilitate repair or replacement of failed parts.An optional bypass is used when some steam flow must be maintained at all times, even during repairs.The bypass line size is usually about half the line size of the pressure regulator, since it will only be opened on an emergency basis, for hopefully a relatively short time.

Figure 2.

Safety is addressed by using a downstream relief valve with a pressure setting of either 5 lbs over the usual downstream pressure, or 5 lbs under the downstream equipment’s lowest pressure rated item.The relief valve is needed to protect the equipment should the pressure regulator fail.Its capacity rating should, at least, match or exceed the maximum flow rate.Note that if the bypass is ever fully opened, close to full upstream pressure will be applied downstream of the regulator.Codes prevent the installation of a shut-off valve between a relief valve and the system it is protecting, so careful adjustment of the manual bypass valve needs to be observed when using the bypass to prevent the relief valve from opening.

Caution:The system should be constantly attended whenever the bypass is opened.Never remove the relief valve and plug the opening!Expensive damage to equipment could result, not to mention a very unsafe and dangerous condition.

A separator is shown as part of the reducing station to insure “dry” steam enters the pressure regulator.“Wet” steam is very detrimental to a pressure regulator.A separator needs to be drained by a trap system.Unfortunately, separators are expensive and are almost never purchased as part of the reducing station.The steam supply line may (should) be dripped before entering the pressure reducing station.When designing and quoting the reducing station, the separator is best quoted as an option.

Gauges are selected according to the pressures being dealt with.The gauges should be mounted on siphon tube assemblies with valves to facilitate quick and easy replacement.

To keep costs down, the strainer and ball valves are the same pipe size as the pressure regulator and should be as close coupled as possible to the regulator.Use close or short nipples.No unions are shown in Figure 2, but should be used as needed.

The regulator’s sensing line should be pitched down slightly so any condensate will drain out of it.It must be at least four feet long and connect into the larger downstream pipe near the outlet end of the pipe.If copper tube is used for the sensing line, care must be taken not to kink the tube, let is sag, or form any trap in its length.

Figure 3 is a picture of what a typical reducing station might look like.Figure 3 shows gate valves instead of better, less expensive, ball valves.

Figure 3.

Often a control valve will be installed directly downstream of the reducing station.

Figure 4 shows such an installation.Chances are the control valve will be the same line size as the pressure regulator.It should be installed downstream of the enlarged downstream piping using eccentrics as shown.The ball valve can be moved downstream of the control valve.If a bypass is used, connect it downstream of the ball valve. As you can see, a steam pressure regulator reducing station is not just a pressure regulator cut into a steam line.The station needs to be carefully designed and properly installed.The entire station should pitch down slightly in the direction of flow.Done correctly, the station will give many years of quiet, satisfactory, performance.